Resonant chamber for microphones



Aug. 15, 195o F. MASSA 2,518,805

RESONANT CHAMBER FOR MICROPHONES Filed Aug. 24, 1945 5g ffy. 7

nnentor 514 w/M/ 1 2 fr 5 f/rnpuEA/cy /N Ka.

Patented ug. l5,

UNITED STATES PATENT OFFICE 2,518,805 REsoNANT CHAMBER FOR monoPHoNEs Frank Massa, Cleveland Heights, Ohio Application August 24, 1945, seria1N0.12,451

(o1. isi- 0.5)

1 l6 Claims.

My invention is concerned with microphones, and, more particularly, with means for varying the frequency response characteristic of a microphone.

In certain applications, of which a vacuum tube hearing aid is an example, it is desirable to provide adjustment for the frequency response characteristic or the system in order to permit the user of the instrument to adjust the characteristic to suit his needs best. Heretofore, the means employed for permitting an individual to obtain the frequency response best suited for his individual requirements have been to provide a number of receivers having different response characteristics and also variable electric circuit elements to cause changes in the response characteristic of the amplifier.

In my invention, I provide an inexpensive means for varying the response characteristic of the microphone which, besides being simpler, cheaper and more flexible than the heretofore used methods, possesses the additional advantage that the signal to noise ratio of the system is improved because I am able to realize an absolute increase in the sensitivity of the microphone over desired portions of the audio-frequency range, and thereby increase the electrical signal at the ampliiler input for a given sound intensity, which serves to reduce the audible background noise resulting from thermal noise or microphonic agitation of the early amplifier stages.

ri'he basic principle of my invention is to mount the microphone Within an enclosure which communicates with the outside atmosphere through an opening; then, by varying the size oi the opening through which sound is admitted to the enclosure or by varying the volume of the enclosure, I am able to increase the sensitivity of the microphone over different portions of the audio-frequency range determined by the size of opening and size of volume or enclosure which are employed.

In one embodiment of my invention, for an improved type of hearing aid unit, I mount the conventional hearing air microphone within a sealed-olic portion of the amplifier housing and provide an opening along the side of the housing over which may be moved an adjustable shutter to ry the amount o opening exposed. By this means, the user of the instrument may adjust the tone quality of his device to suit his needs best.

As generally worn, a hearing aid instrument is placed underneath the outer garments, and for the conventional structure employing a microphone with an open grill, the clothing seals off the openinvs on the iront of the microphone housing, thereby resulting in considerable attenuation of the higher audio Ifrequencies. In one adaptation of my invention for hear-ing aid use, I place the adjustable opening along the edge of 2 the amplifier housing so that when the housing is placed `beneath an article of clothing, the open edge will be more generally unobstructed to the sound approaching the unit.

An object of my invention is to vary the frequency response characteristic of a microphone by simple external adjustable mechanisms.

Another object of my invention is to increase the sensitivity of a microphone over any portion of a desired range of audio frequencies.

Still another object ofmy invention is to vary the response characteristic of a microphone by mounting it within an enclosure which communicates to the free atmosphere through an adjustable opening.

A further object of my invention is to vary the response characteristic of a microphone by mounting the microphone within an enclosure of adjustable volume which communicates to the iree atmosphere through an orifice.

A still further object of my invention is to provide simple tone adjustment means in a hearing aid instrument by mounting a microphone Within an enclosed portion of the hearing aid instrument and providing an adjustable opening for communicating: between the enclosure and the out- Side, atmosphere.

Another object of my invention is to provide a hearing aid device whose sensitivity over any portionl of a desired range of audio frequencies may be enhanced to fit the hearing aid to the users particular requirement.

It is also an object of my invention to provide a hearing aid device whose sensitivity over a portion of its frequency range may be changed from time to time as the Wearers hearing changes, thereby saving the wearer from purchasing a number of the devices.

Still a further object of my invention is to provide a hearing aid device which may be manufactured asl a standard item and which the salesman o rv the wearer himself` may easily adjust to compensate for the wearers particular hearing defect.l

Other objects and advantages of my invention will become evident by reading the specifications which follow. The novel features that I consider characteristic o i my invention are set for-th with particularity in the appended claims. The invention, itself, however, both as to its organization and method of operation, as well as advantages thereof, will best be understood from the following description or several embodiments thereof', when read in connection with the accompany-ing drawings, in which- Fig. l shows a microphone mounted within an enclosure which is provided with an adjustable 'opening for communicating from the enclosure to the outer atmosphere.

Fig. 2 is a section taken along the line 2-2 of Fig. 1.

Fig. 3 shows a partially ycut-away view of another arrangement for realizin-g the principle of my invention in which a microphone is-mounted within an enclosure of adjustable volume which communicates to the outer atmosphere through a xed orice.

Fig. 4 is a section taken along the line 4--4 of Fig. 3.

Fig. 5 is a schematic view of a hearing aid instrument with the front cover removed and showing the location of the microphone and a variable shutter mechanism for adjusting the opening which communicates from the outer atmosphere to the enclosure in which the microphone is mounted. Y

Fig. 6 is a side view of the hearing aid unit indicated in Fig. 5 with the front cover attached.

Fig. 7 is a graph showing the changes in the frequency response characteristic produced by varying the position of the shutter in either Fig. 1 or Fi-g. 5.

Referring more specically to Fig. 1 and t0 Fig. 2, the reference character I indicates a conventional small pressure microphone as generally employed in hearing aid instruments. The microphone is resiliently mounted on a back plate 2 by means of flexible rubber pads 3, which may be cemented or otherwise affixed to the back plateV 2 and to the housing of the microphone I. The back plate 2 is attached to a cup-shaped housing 4 by means of the screws 5. Into the at or bottom face of the housing 4 is cut a semi-circular opening 6 which is shown dotted in Fig. 1. To

the flat face of the housing 4 is attached a ro` ytatable disc 'I which is held in place by the rivet 8. Into the disc I is cut a semi-circular opening 9 which may be brought into alignment with the opening 6. rivet 8 with respect to the housing 4, the amount of opening which is exposed through the housing 4 may be controlled. As the amount of opening Vis varied, the microphone I will find itself ef- Vshutter 1, the microphone response will correspondingly change in accordance with the position of shutter 1. Some actual experimental data showing the measured changes in the response of f a microphone under certain conditions of enclosed volume and orice area will be presented in connection with Fig. 7. Although it is not indicated in Figs. 1 and 2, I may add a sheet of sound absorbing material such as felt over the opening 6 if I wish to damp the acoustical resonance and reduce the magnitude of the increase in the peak sensitivity of the microphone in the resonance frequency range determined by the microphone enclosure volume and the size of the adjustable opening.

In Fig. 3 and Fig. 4 is shown an alternative arrangement for producing the same eiect as obtained in the system of Figs. 1 and 2. The microphone I is mounted on flexible rubber pads 3 in the same way as described in connection with Fig. 2. The microphone is attached to the back plate I which, in turn, is secured to the substantially cup-shaped housing II by means of the screws I2. Through the housing II is a VAs the disc I is rotated about the4 Since this region Y.

screen-covered opening I3 which provides communication from the outer atmosphere to the volume enclosed by the housing II and back plate I0. An adjustable end plate I4 which may be moved as high as the position I4 completes the assembly. rIhe end plate I4 slidably fits within the housing so that frictional forces hold it in any place to which it is moved, or alternatively, set screws or a notch and spring arrangement somewhat similar to the notch and spring device shown in Fig. 5 may be used to hold the end plate I4 within the housing. As the end plate I4 is moved within the housing, the volume of the enclosure will be varied, which will result in a variation in the frequency response characteristic ofthe microphone in a manner similar to that produced by varying the shutter opening in Fig. l, and, although it is not illustrated, the device shown in Figs. 3 and 4 may have its opening I3 variable in size as has been described in Figs. 1 and 2.

Figs. 5 and 6 show the schematic embodiment of my invention in connection with a vacuum tube hearing aid instrument. The housing of the instrument consists of a case I5 and a front cover I6 which is shown in Fig. 6 but which is removed in Fig. 5. A ridge 3l is provided to divide the housing I5 into two portions. A mating ridge, not shown, is provided in the cover piece I6 so that when the cover is assembled in place, the housing I5 will be divided into two separate compartments, for example, a top compartment and a bottom compartment. The microphone I is mounted in the top compartment, as indicated in Fig. 5. The reference character I1 illustrates a volume control which may be located, as shown, along the top edge of the housing, A small ridge I8 is provided in the housing I5 so that a guide track I9 will be provided for the shutter 20. A similar ridge and guide for the shutter 20 is also provided along the inside edge of the cover piece I6 so that at nal assembly the shutter 20 is eifectively secured between two parallel guides. A number of notches 2I are provided, as shown on the back of the shutter 20, and a spring 22 which is riveted to the wall of the ridge 3| acts as a locking device to hold the shutter in any desired notched position. A rectangular opening 23 exists along one edge of the housing I5, as shown. A smaller, similar rectangular opening 24 is provided along the top edge of the housing I5. The size of the opening 24 is made such that when the shutter 20 is raised to close completely the opening 23, the opening 24 will provide increased microphone sensitivity at the lowest frequency desired in the hearing aid instrument. As the shutter 2B is lowered successively to cause the opening 23 to become larger and larger, the high-frequency sensitivity of the microphone will continue to increase more and more, thereby resulting in a variable tone quality which can be set at the will of the user. Although a manual adjustment for the shutter 20 has been illustrated, I may also provide other means for adjusting the position of the shutter 20 that would necessitate the use of special tools such as, for example, a locking set screw mounted into the housing I5 and adjustable for providing pressure against the shutter 20 to hold it in a xed position. With such an alternative means for setting the size of the opening 23, the adjustment may be made by a skilled technician to best t the individuals hearing loss characteristic. This adjustment may be repeated from time to time whenever the individual wishes to be retted The cover piece i6 is made without any openings whatsoever. The only communication between the microphone and the outer atmosphere is through the side openings 23 and 24. This construction was chosen so that when the hearing aid instrument is worn underneath a coat or other garment, there will be less obstruction than would be the case were the openings kept on either the front or rear surface of the instrument. Reference characters 25, 2S, 21, and 28 are schematic representations of circuit elements such as vacuum tubes or resistors that may form the amplifier circuit of the hearing aid instrument. Electrical connection may be established from the upper compartment of Fig. 5 to the lower compartment oi Fig. 5 through suitable terminals which may be provided through the wall 3|. In the lower compartment of Fig. 5, the reference characters 28 and 30 are schematic representations of other components or" the electronic device such as, for example, the batteries which operate the instrument. These batteries may be mounted on spring clips, not shown, and a suitable opening, not shown, may be provided on the rear of housing l5 to permit ease in their replacement.

Fig. 7 shows the measured response characteristie of a small pressure microphone mounted inside an enclosure having a volume yof 1'1/2 cubic inches. A communicating slit wide was provided along one side of the housing. Curve A shows the measured response characteristic of the microphone in free space. Curve B shows the response of the microphone when mounted in the enclosure with the slit opening adjusted to a length of approximately 1A, inch. Curve C shows the measured characteristic when the slit opening was increased to approximately one-half inch. Curve D shows the characteristic when the slit opening was increased to approximately one inch, and curve E shows the characteristic measured with the slit opening approximately 1% inches long. In each case, there is an appreciable increase in the absolute sensitivity of the microphone over certain frequency regions, and the wide range of frequency over which the system is adinstable by the simple variable opening described will permit a wide choice of tone variations in a single instrument to permit easy tailoring of the acoustic characteristic to best lit the hearing loss of any individual. The response curves shown in Fig. 7 were taken without any acoustic damping material placed over the adjustable opening of the microphone enclosure. If acoustic damping material were placed over the opening, as previously mentioned, the tops of the peaks of the various curves shown in Fig. 7 would be rounded oli to result in less gain in the absolute peak sensitivities at the various frequencies.

The use `of this invention will provide great flexibility in tone control setting in a hearing aid instrument and because of the fact that the absolute sensitivity of the microphone is actually increased by my invention, an added advantage results over other means of electrical compensation in that the signal to noise ratio of the instrument is improved.

Although I have chosen only a few specic examples to illustrate the basic principles of my invention, it will be obvious to those skilled in the art that numerous departures may be made from the details shown, and I, therefore, desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit vof the appended claims.

I claim as my invention:

1. In an electronic hearing aid system of the wearable type a thin housing structure comprising a walled enclosure of predetermined volume having an orice in a wall thereof, said enclosure: defining an acoustic resonant system, the cube root of the magnitude of the unoccupied portion of the volume within said enclosure being less than 1A; wavelength of the resonant frequency of said enclosure in the range 800 cycles per second to 2500 cycles per second, a pressure-actuated microphone mounted within said enclosure, and adjustable shutter means associated with said housing for varying the size of said orifice whereby the sensitivity of the microphone may be increased over predetermined portions of the frequency region 800 cycles per second to 2500 cycles per second.

2. The invention set forth in claim 1, further characterized in that said orice is placed along one edge of said thin housing and said associated mechanical means for varying the size of said orice includes a slidable wall portion attached to said housing, and means for selectively holding said slidable wall portion in any one of a plurality of positions.

3. In combination, a pressure-actuated microphone suspended within a walled enclosure of predetermined volume having an orice in a wall thereof, said enclosure deiining an acoustic resonant system, the cube root of the magnitude of the unoccupied portion of the volume within said enclosure being less than 1A wavelength of the resonant frequency of said enclosure in the range 800 cycles per second to 2500 cycles per second, the surface area of at least one wall of said enclosure exposed to said microphone being variable.

4. The invention set forth in claim 3, further characterized in that one wall of said enclosure is movable with respect to the other walls of said enclosure whereby the volume within which said microphone is suspended may be varied.

5. The invention set forth in claim 3 further characterized in that adjustable shutter means is associated with said enclosure and is arranged to permit the closing oi of a portion of said orifice.

6. The invention described in claim 3, characterized in that acoustic damping material is provided over said orice.

FRANK MASSA.

REFERENCES CITED The following references are of record in the iile of this Ipatent:

UNITED STATES PATENTS Number Name Date 1,372,162 Petri-Palmedo Mar. 22, 1921 1,463,372 Reynolds July 31, 1923 1,715,831 Hahnemann June 4, 1929 1,734,944 Green Nov. 5, 1929 1,884,724 Keller Oct. 25, 1932 1,915,358 Giles June 27, 1933 1,918,839 De Puy July 18, 1933 1,967,223 Bostwick July 24, 1934 2,162,270 Mott June 13, 1939 2,196,342 Ruttenberg Apr. 9, 1940 2,224,974 Lybarger Dec. 17, 1940 2,390,794 Knight Dec. 11, 1945 2,404,877 Horlacher July 30, 1946 FOREIGN PATENTS Number Country Date 281,866 Great Britain Dec. 15, 1927 

